Hydrocracking heavy petroleum cuts is a very important process in refining that allows to produce, from a surplus of hardly upgradable heavy feeds, lighter fractions such as gasolines, jet fuels and light diesel fuels sought by refiners so as to adjust their production to the structure of the demand. Some hydrocracking processes also allow to obtain a highly purified residue that can provide excellent bases for oils. In relation to catalytic cracking, the interest of catalytic hydrocracking is to provide middle distillates of very good quality. On the other hand, the gasoline produced has a much lower octane number than the gasoline obtained from catalytic cracking.
The flexibility of hydrocracking results from three main elements, i.e. the operating conditions used, the types of catalyst used and the fact that hydrocracking hydrocarbon-containing feeds can be carried out in one or two stages.
The hydrocracking catalysts used in hydrocracking methods are all of bifunctional type, combining an acid function and a hydrogenizing function. The acid function is provided by supports whose surface areas generally range from 150 to 800 m2.g−1 and having a superficial acidity, such as halogenated aluminas (notably chlorinated or fluorinated), combinations of boron and aluminium oxides, amorphous silica-aluminas and zeolites. The hydrogenizing function is provided by either one or more group VIB metals of the periodic table, or by a combination of at least one group VIB metal of the periodic table and at least one group VIII metal.
The balance between the acid and hydrogenizing functions is one of the parameters that govern the activity and selectivity of the catalyst. A weak acid function and a strong hydrogenizing function give weakly active catalysts, operating at a generally high temperature (greater than or equal to 390°-400° C.), and at a low space velocity (the LHSV expressed in volume of feed to be treated per unit volume of catalyst and per hour is generally less than or equal to 2), but exhibiting a very good middle distillate (jet fuels and diesel fuels) selectivity. On the other hand, a strong acid function and a weak hydrogenizing function give active catalysts, but with lower middle distillate selectivities.
A conventional type of hydrocracking catalyst is based on moderately acidic amorphous supports such as silica-aluminas for example. These systems are used to produce good-quality middle distillates and possibly oil bases. These catalysts are for example used in single-stage methods. The drawback of these catalysts based on amorphous supports is their low activity.
Catalysts comprising for example a Y zeolite of FAU structural type, or catalysts comprising for example a zeolite of beta type, have a higher catalytic activity than silica-aluminas, but lower middle distillate (jet fuels and diesel fuels) selectivities. This difference is due to the acid site strength difference on the two types of material.
The modification of zeolites through the deposition of compounds containing at least one molecular compound comprising at least one silicon atom has been widely studied in the past. Examples thereof are, among others, U.S. Pat. No. 4,402,867, which describes a method of preparing a zeolite-based catalyst comprising a stage that consists in depositing in aqueous phase at least 0.3 wt. % amorphous silica in the pores of the zeolite. U.S. Pat. No. 4,996,034 describes a method of substituting aluminium atoms present in a zeolite framework for silicon atoms, said method being carried out in a single stage in an aqueous medium using fluorosilicate salts. U.S. Pat. No. 4,451,572 describes the preparation of a zeolitic catalyst comprising a stage of deposition of organosilicic materials in vapour or liquid phase, the zeolites concerned being wide-pore zeolites, in particular the Y zeolite. The zeolite treated with this method however contains more than 23% alkaline cation Na+ in the structure of the zeolite after modification.